Delta

A river delta occurs as a “wetland that forms as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river,” and may “also empty into land, ”though this is rare [1]. The river that discharges in river mouths to form deltas is always carrying an amount of sediment, silt, gravel, or clay along with it that must have a “rate of sediment supply exceeding the rate of sediment removal by waves and tidal currents,” where the “forces on the formation process” are either “river-, wave-, or tide-dominated” [2], [3]. Rivers that flow into deltas often are not fast flowing waters, typically slow flowing and calmer as the “river nears its mouth, or the end,” which leads to the formation of a “deltaic lobe,” or mouth bar, a “network of smaller, shallower channels branching off from the mainstream” and the “sediment falling to the river bottom,” and at this lobe an expanse of sediment is noticeable around the smaller, shallower streams [1].

Deltas vary mostly by what type of sediment they carry with them, where the sediment is crucial to the deltas and make them an important yet overlooked ecosystem. As ‘Restore the Mississippi River Delta’ states, “fine sediments like clays and muds are highly mobile and tend to travel the farthest distance; they also make up nearly 80% of the sediment load in most rivers." Other ecosystems, such as wetlands and mangroves, depend on a stable delta, and one of the reasons they can serve as a base for these ecosystems is because soils around deltas are very rich and highly silty, supporting great plant growth. This also makes them an important resource for multiple different animals, such as fish, birds, and insects [4].

Observing the appearance of river deltas, we generally expect to observe the delta to be “shaped like a triangle,” as the “term delta comes from the upper-case Greek letter delta (Δ),” and these deltas “are called arcuate (arc-like) deltas." Other types of deltas include the “cuspate delta, which is more pointed than the arcuate delta, and is tooth-shaped,” the “bird-foot delta” which “has few, widely spaced distributaries, making it look like a bird's foot,” and the “inverted delta” which is very uncommon with its “distributary network” being “inland, while a single stream reaches the ocean or other body of water” [1].

References

[1] National Geographic Society. (2013, December 12). Delta. Retrieved (2021, July 21), from https://www.nationalgeographic.org/encyclopedia/delta/

[2] National Park Service (NPS). (2019, March 20). Delta Landforms. Retrieved (2021, July 21), from https://www.nps.gov/articles/delta-landforms.htm

[3] Proceedings of the National Academy of Sciences of the United States of America (PNAS). (2007, October 23). Modeling River Delta Formation. Retrieved (2021, July 21), from https://www.pnas.org/content/104/43/16804

[4] Restore the Mississippi River Delta. (2017-2021). Anatomy of a Delta: The Foundation of New Land. Retrieved (2021, July 22), from https://mississippiriverdelta.org/learning/anatomy-of-a-delta-the-foundation-of-new-land/